Vortex shedding flow meters have been used for many applications and are able to measure the flow rates of a variety of fluids, including steam, liquids, and gases. A vortex shedding flow meter operates on the principle that a bluff body, when placed in a moving fluid, produces an alternating series of vortices at a frequency that is directly related to the velocity of the moving fluid. Some vortex shedding flow meters detect the frequency of the shed vortices, thus the flow rates, by having a vane that is in communication with a piezoelectric material, positioned downstream from the bluff body. As the vortices pass over the vane, alternating lateral forces deflect the vane one way and then the other creating a surface charge about the piezoelectric material. The surface charge of the piezoelectric material is a function of the strain on the vane and therefore the velocity of the fluid may be measured.
Based on the design of current vortex shedding flow meters, however, the piezoelectric materials, are susceptible to producing charge not only when there is a deflection of the vane but also through turbulence and noise within the measured system, yaw (strain in the direction of the flow) due to drag, and vibrations.
There are a number of selection criteria for an appropriate piezoelectric material, including sensitivity, dynamic range, signal-to-noise ratio, temperature and cost. Sensitivity is directly related to the piezoelectric coefficient of the material. Dynamic range is a function of both sensitivity and mechanical robustness, meaning the material must generate a usable charge signal at low flow as well as remain mechanically sound at maximum strains, often a million times greater. Maximizing signal-to-noise requires that the piezoelectric material only respond to the specific mechanical strain vector being measured and reject all others. Further, bulk temperature and electromagnetic effects such as pyroelectric and ferromagnetic noise should preferably have little effect on the piezoelectric material. Accordingly, there are a number of factors that should be considered before an appropriate and effective piezoelectric material is found.